The instant invention relates to means for sensing the acceleration profile of an object, such as a motor vehicle.
The prior art teaches magnetically-biased acceleration sensors, or accelerometers, comprising a housing having an inertial or "sensing" sensing mass within an internal passage which is magnetically biased towards a first end of the passage by a magnet secured to the housing. When the housing is subjected to an accelerating force which exceeds the magnetic bias, the sensing mass begins to move from its rest position proximate to the passage's first end toward the passage's other end, with such movement preferably being retarded by a suitable damping means. Where the acceleration input to the housing is of sufficient magnitude and duration to displace the sensing mass to the other end of the passage, the sensing mass triggers a switch means in the sensor, as by bridging a pair of electrical contacts, to actuate an instrumentality connected with the switch means, such as a vehicle passenger safety restraint. In this manner, the sensor mechanically integrates the acceleration input to the housing.
In U.S. Pat. No. 4,827,091, I teach an accelerometer having a magnetic sensing mass which magnetically interacts with a magnetically-permeable element secured to the housing so as to be magnetically biased toward the passage's first end. When the housing is subjected to an acceleration sufficient to overcome the magnetic bias, the sensing mass is displaced towards the contacts at the other end of the passage. In accordance with another feature disclosed in my '091 patent, any sensing mass displacement is damped by virtue of the magnetic interaction of the sensing mass with one or more electrically-conductive nonmagnetic rings which encompass the passage, while the switch contacts at the other end of the passage move longitudinally in response to temperature in order to compensate for the effects of temperature on the magnetic damping effect.
Unfortunately, while my '091 accelerometer patent compensates for temperature effects on the magnetic damping through temperature-responsive longitudinal movement of the contacts, the accelerometer does not otherwise compensate for the effects of temperature on the magnetic flux generated by the sensing mass as it affects the sensor's threshold magnetic bias. Thus, as the magnetic flux generated by the sensing mass reversibly decreases with increasing temperature, the threshold magnetic bias is correspondingly decreased, with the attendant risk that the safety restraint controlled by the sensor will be triggered by a relatively low acceleration input.
In U.S. Pat. No. 4,922,065, I teach a magnetically-biased accelerometer similar to that of my '091 patent, wherein the first end of the passage is itself defined by a stop which moves longitudinally of the passage in response to changes in the operating temperature of the sensor, whereby the separational distance between the magnetic sensing mass and the washer and, hence, the nominal threshold magnetic biasing force on the sensing mass when positioned against the stop, are adjusted to compensate for the effects of temperature on the magnetic flux generated by the magnetic sensing mass.
U.S. Pat. No. 4,477,732 to Mausner teaches an accelerometer utilizing a spring-restrained inertial mass which is deflected relative to a fixed reference point when the mass is subjected to a particular acceleration force. Mausner discloses the placement of a photoconductor on the inertial sensing mass for receiving a beam of light from a light source. A set of three additional photoconductors are placed longitudinally adjacent the sensing mass. A first one of the three photoconductors aligns with the sensing mass photoconductor to generate a signal when the sensing mass is in a first rest position. The other two photodetectors are positioned to generate signals when the sensing mass exhibits maximum displacement in a negative sense (deceleration) or a positive sense (acceleration) respectively.
However, Mausner does not teach any testing or calibrating capability. Additionally, the accelerometer of Mausner requires several optical fibers and photoconductors per single accelerometer. Utilizing a number of these accelerometers in a crash discrimination system would be very costly due to the numerous dedicated optical components. Reliability of the system is also reduced with the increase in optical fibers required by the system.
It is further noted the accelerometer of Mausner only provides a "go/no go" detection mode. In other words, the accelerometer output signal only indicates if the sensing mass has not moved (no collision), or if the sensing mass has been maximally displaced by the detected acceleration (collision). Thus, Mausner cannot provide an accelerometer output proportional to any sensing mass movement less than maximum displacement.
Finally, it is noted that accelerometers are frequently deployed in pairs in the interest of increased reliability, e.g., a first sensor having a relatively low acceleration threshold serves to "arm" a second sensor having a relatively high acceleration threshold tailored to the particular application involved. However, in the event that the second, high-threshold sensor fails in the "closed" condition, i.e., incorrectly indicates an acceleration condition necessitating the deployment of the instrumentality controlled thereby, any acceleration exceeding the low acceleration threshold of the first, "safing" sensor will cause the deployment of that instrumentality. A graphic illustration of this condition is the deployment of an air bag upon encountering a pothole subsequent to the failure of the high-threshold sensor. It is, therefore, highly desirable to be able to spontaneously increase the biasing force on the sensing mass of the arming sensor and, hence, its acceleration threshold, upon an indication that the high-threshold sensor has "failed closed."
Thus, in accordance with another feature disclosed in my '091 patent, a sensor employing a magnetic sensing mass may further include an electrical coil encompassing the passage which, when energized by the delivery of a first direct current therethrough, effects the displacement of the sensing mass to the passage's other end and against the contacts to confirm the operability of the sensor. Alternatively, a second direct current may be delivered through the coils in the reverse direction, whereby the magnetic bias on the sensing mass is controllably increased.